The invention relates to a laser glass with particularly low threshold energy for stimulated emission and negative temperature coefficient of refractive index.
In another aspect, this invention relates to a method for making neodymium-doped phosphate glass suitable for fabrication into rods for lasers.
In another aspect, this invention relates to lasers in which the active medium is a neodymium-doped phosphate glass.
The term "laser" is an acronym for "light amplification by the stimulated emission of radiation". The amplified light may include visible light, as well as the infrared and ultraviolet portions of the frequency spectrum and is within the wavelength band of 10.sup.2 to 10.sup.6 Angstroms.
Operation of a laser depends on interaction between matter and radiation which occurs when matter, that is, atoms or molecules, absorbs or emits photons. Atoms or molecules in the ground or non-radiating state, i.e., atoms or molecules which do not emit energy, have an energy of a fixed amount or level. If an atom in the ground state is excited, i.e., interacts with an incident photon, the atom becomes in a higher or excited state, as long as the energy of the photon is at least equal to the energy difference between the ground and excited states. When the energy level of an atom or molecule is changed from a higher energy state to a lower energy state, a photon of radiation may be emitted. The energy of the emitted photon is equal to the difference in the energy between these states.
An atom in an excited state can emit a photon spontaneously and revert to its ground state or some intermediate lower state. However, while the atom is still excited, it can be stimulated to emit a photon by interaction with an incident photon of energy equal to that of the photon which would be emitted spontaneously. The result is that the incoming photon or electromagnetic radiation wave is augmented by the one given up by the emitting atom. This released wave falls in phase with the wave which triggered its release, that is, an amplifying action called stimulated emission is brought about.
Usually, more atoms are in lower energy states than in the various higher energy states. But, in a laser, the energy level distribution of electrons is altered by a process called "pumping," so that there are more atoms in higher levels than in lower levels. Pumping is the injection of some form of energy, e.g., electromagnetic energy, into a population of atoms, a significant number of which absorb energy and are elevated to excited states. Thus, incident photons of the lowest energy can produce more downward than upward energy level transitions and stimulated emission can result.
In a laser, a suitable active material, which can be a solid, a liquid or a gas, is enclosed in a cavity resonator which has at least two separated reflecting walls. A wave starting anywhere within the cavity grows in amplitude until the wave reaches either wall and is thereupon reflected back into the medium. Although there are, in practice, losses from imperfect reflections, absorption and scattering, a wave will build up in the resonator if the amplification by stimulated emission is larger than the losses. In the present invention, the active material is a glass rod and the two separated reflecting walls are mirrors.
For constant operating conditions with rapid impulse sequence, it is desirable to have a laser glass with low threshold energy and negative temperature coefficient of refractive index. In the processing of materials and range-finders, it would be desirable to produce laser pulse sequences of about one or more pulses per minute. It is also important in this connection that the pulses be of equal strength.
If a laser material has a positive temperature coefficient of refractive index, the laser beam becomes increasingly divergent as operation proceeds owing to heating of the laser rod by pumping radiation. Temperature differences between the marginal zone and the interior of the rod can also be caused by this radiation. Thus, after the initiation of the light pumping, the outer zone of the rod can become warmer than the inner zone. When the temperature coefficient of the laser glass is positive, the optical path length in the marginal zones becomes greater. This causes an effect which is similar to the introduction of a concave lens into a parallel beam so that the beam emerging from the rod becomes divergent. Generally, crystals and glasses have a positive temperature coefficient, so that their use in laser rods is unsatisfactory.
However, there are glasses with negative temperature coefficients. The optical path length is then less strongly influenced by the temperature. The variation of the optical path length .DELTA.s caused by a temperature difference .DELTA.T in the rod is: ##EQU1## when a) the mirrors are vapor deposited on the end faces of the rod, and b) when the resonator mirror is erected separately from the laser rod, respectively. In these formulae:
.alpha. = thermal expansion PA1 n = refractive index PA1 L = rod length. PA1 a length of about 75 to 300 mm and PA1 a diameter of about 4 to 20 mm.
It is seen from these formulae that the path length difference is smaller when the temperature coefficient of the laser rod is negative.
Apart from the low threshold energy of the laser effect and the negative temperature coefficient of the refractive index, the following properties are also required for active solid laser body material: good chemical resistivity; low thermal expansion, no tendency of the melt to crystallize and a small tendency to striation; and freedom from particles which absorb and could lead to destruction of the glass with excessive pulse generation.
It is known that phosphate glasses have a relatively low threshold value for the laser effect. See, for example, O. K. Deutschbein, C. C. Pautrat and I. M. Svirchevsky, Revue de Physique Applique, Volume 2, 1967, 29-37; DePaolis, et al., U.S. Pat. No. 3,250,721; Hirayama et al., U.S. Pat. No. 3,549,554; and Buzhinsky et al., U.S. Pat. No. 3,846,142. However, phosphate glasses usually have one or more of the following disadvantages: low chemical stability or even high solubility in water; high thermal expansion, up to 190 .times. 10.sup.-.sup.7 /.degree. C.; and inhomogeneity in pieces of relatively large dimensions. However, these objections are avoided with the glasses of this invention.